feat: add access group and keychain synchronization

[cbaker6]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

Items in keychain currently can't be synced across apps or groups.

Related issue: #n/a

Approach

Follow Apple's guide to add an access group and enable synchronizing keychain across devices. The method below has been added:

/**
     Sets all of the items in the Parse Keychain to a specific access group.
     Apps in the same access group can share Keychain items. See Apple's
      [documentation](https://developer.apple.com/documentation/security/ksecattraccessgroup)
      for more information.
     - parameter accessGroup: The name of the access group.
     - parameter synchronizeAcrossDevices: **true** to synchronize, **false** to disable synchronization.
     - throws: An error of type `ParseError`.
     - returns: **true** if the Keychain was moved to the new `accessGroup`, **false** otherwise.
     - warning: Setting `synchronizeAcrossDevices == true` requires `accessGroup` to be
     set to a valid [keychain group](https://developer.apple.com/documentation/security/ksecattraccessgroup).
     */
    @discardableResult static public func setAccessGroup(_ accessGroup: String?,
                                                         synchronizeAcrossDevices: Bool) throws -> Bool

// Call the method
do {
  try ParseSwift.setAccessGroup("name.of.access.group", synchronizeAcrossDevices: false)
} catch {
  // Handle error
}

All Keychain items can be synced except for ParseInstallation and ParseVersion as both of these are unique to it's respective install.

TODOs before merging

• [x] Add tests
• [x] Add entry to changelog
• [x] Add changes to documentation (guides, repository pages, in-code descriptions)

[parse-github-assistant[bot]]

Thanks for opening this pull request!

• 🎉 We are excited about your hands-on contribution!

[codecov[bot]]

Codecov Report

Merging #378 (b530234) into main (89e5e69) will increase coverage by 0.23%. The diff coverage is 98.90%.

@@            Coverage Diff             @@
##             main     #378      +/-   ##
==========================================
+ Coverage   89.06%   89.29%   +0.23%     
==========================================
  Files         156      157       +1     
  Lines       14778    14970     +192     
==========================================
+ Hits        13162    13368     +206     
+ Misses       1616     1602      -14     

Impacted Files	Coverage Δ
Sources/ParseSwift/Storage/ParseStorage.swift	82.14% <ø> (ø)
Sources/ParseSwift/Types/ParseACL.swift	93.58% <ø> (ø)
Sources/ParseSwift/Storage/KeychainStore.swift	97.86% <98.40%> (+2.21%)	:arrow_up:
Sources/ParseSwift/Objects/ParseInstallation.swift	85.27% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseObject.swift	87.29% <100.00%> (ø)
Sources/ParseSwift/Objects/ParseUser.swift	86.34% <100.00%> (+0.19%)	:arrow_up:
Sources/ParseSwift/Parse.swift	91.28% <100.00%> (+2.61%)	:arrow_up:
...es/ParseSwift/Protocols/ParseHookRequestable.swift	100.00% <100.00%> (ø)
Sources/ParseSwift/Types/ParseConfig.swift	100.00% <100.00%> (+1.09%)	:arrow_up:
...es/ParseSwift/Types/ParseKeychainAccessGroup.swift	100.00% <100.00%> (ø)
... and 8 more

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